In the continuous casting of steel a cast steel strand continuously leaves the casting mold with a relatively thin skin of solidified steel containing molten steel, the strand traveling through a series of rollers which support the skin on opposite sides of the strand, cooling progressively causing the skin to thicken by solidification of the molten steel until a solid steel strand is obtained.
It is desirable to stir the molten steel within the skin, for example, to prevent the formation of pipe and dentrites.
It is known that molten steel in a strand during continuous casting can be stirred by one or more stirrers comprising induction coils supplied with multi-phase AC having frequencies within the range of 10-60 Hz.
The Alberny et al. U.S. Pat. No. 3,882,923 discloses the above type of stirring, the induction coils being positioned inside of the rollers supporting the skin. This patent recomments the use of multi-phase AC having the frequencies of 10-60 Hz.
The stirring force obtained with a stirrer with a given linear current density can be shown to be proportional to the frequency if the influence of the induced current on the distribution of the magnetic field is neglected. Following this rule, the frequency should be chosen as high as possible in order to obtain as big a force as possible with a given linear current density.
However, when taking the influence of the induced currents into account when deriving a formula for stirring force, one finds that the proportionality between force and frequency is only valid up to a certain frequency. With increasing frequency the force will reach a maximum and thereafter decreases.
Another phenomenon will also be clear in such a deeper derivation: the volume force density will decrease faster with increasing perpendicular distance from the surface of the strand than a simpler derivation will indicate. This is important in the case of stirring the molten metal inside the skin of a strand during continuous casting. It is also important in a case when the induction stirrer is placed inside steel rollers with a considerable wall thickness.